The Rochester Epidemiology Project (REP) is a medical records linkage system that entails surveillance of the care delivered to the residents of Rochester and Olmsted County, Minnesota. This unique population-based data infrastructure provides access to the detailed inpatient and outpatient medical records of the providers of health care to local residents. The information contained in these records represents the foundation for hundreds of epidemiologic studies that provide insight into the frequency, etiology, natural history and outcomes of heart disease, stroke, dementia, Parkinson's disease, cancer, diabetes, digestive disorders, osteoporosis, arthritis and many other diseases in the population. Moreover, this infrastructure allows the conduct of these studies at a fraction of the cost that would be realized in its absence. Since the infrastructure was first organized for the entire population in 1966, 1198 publications have been generated, including 378 in the most recent 4 years. Although Mayo Clinic supports the indexing of diagnoses and procedures in its own records at no cost to this project, ongoing extramural support is needed to collect information on diagnoses and procedures from the non-Mayo providers and to integrate this information at the patient level. It is this portion of the infrastructure that makes the REP a population-based resource. Specifically, continued funding is needed for 1) maintaining the current infrastructure by continuing to identify individuals across the records of local providers of care, continuing to collect medical event data on Olmstead County residents from the non-Mayo providers of care and continuing the accession of information about medical care to the relational database; 2) enhancing the infrastructure for the conduct of genetic epidemiologic studies by collecting information on family history and family structure of local residents, developing an inventory of biological specimens collected from Olmsted County residents and collecting information on risk behaviors, signs and symptoms; and 3) enhancing the logistic and analytic support for Rochester Epidemiology Project studies generally. These activities are indispensable to the continued support of the large body of epidemiologic research that is being conducted in this population by investigators from Mayo Clinic and other centers throughout the country, including 30 separate studies funded by the National Institutes of Health through the previous grant period. Although the basic data system has worked well, these aims will allow the applicants to extend the capabilities of this research infrastructure and insure its future usefulness.